The present preferred embodiment concerns a method to activate a processing device which processes the surface of a moving web-shaped work piece at regular intervals, wherein a predetermined duration passes between an activation signal of the processing device and the action of the processing on the surface of the web-shaped work piece.
In particular, the preferred embodiment concerns a method to print a recording medium by means of an inkjet print head. An inkjet print head sprays small droplets of printing ink in the direction of the recording medium. These ink droplets require certain duration in order to strike the recording medium. This is the predetermined duration that passes between the activation signal of the print head and the action of the processing on the surface, namely the impact of the ink droplet on the recording medium.
In methods in which multiple subordinate workflows are to be interleaved with spatial accuracy on a web-shaped work piece, portions that cannot vary or cannot be varied corresponding to the movement velocity of the work piece produce a phase shift that varies with increasing velocity. This leads to a spatial deviation, and therefore to positioning, register or mounting deviations, for example.
In inkjet printing, the drop firing point in time is synchronized with the movement of the recording medium. The drop strikes the resting recording medium at a specific location of the web, in contrast to which a deviation corresponding to the web velocity arises at the maximum velocity of the recording medium. This deviation amounts to the web velocity multiplied by the clearance of the inkjet print head, divided by the drop velocity.
If the velocity is constant, a constant offset results that can be statically compensated. The print heads of the individual colors can thus be activated with corresponding compensation, and a print image that is in register can be generated. However, if printing takes place during an acceleration ramp, this offset builds gradually from zero to a maximum and produces a distortion of the print image. The print heads of the different colors are arranged at different locations, such that individual regions of the print image are subject to different distortions with regard to the different color separations. Significant register errors hereby result that far exceed the tolerance values allowed in practice.
In conventional printing devices, it is therefore for the most part not possible to implement a color printing during an acceleration or slowing phase.
A method for ink drop firing time control for an inkjet print device arises from U.S. Pat. No. 6,361,137 B1. In this method, the print head is moved by means of a carriage over the recording medium. Since the movement of the carriage superimposes with the movement of the ink fired from the print head, the ink is not fired vertically towards the recording medium but rather strikes the recording medium at an angle that depends on the velocity of the carriage. In this method, if a compensation of the movement of the print head also takes place, the posing of the problem explained above—in which a processing device must be synchronized with the movement of a web-shaped work piece—is different. This in particular applies if multiple processing devices act at different points of the web-shaped work piece.
Furthermore, a device and a method for scanning a web movement to control a processing process in which an incremental sensor is used to scan the web movement are disclosed in the subsequently published Patent Application DE 10 2009 038480 by the applicant. Given a backward movement (not provided per se) of the material web, the output of the signals generated by the incremental sensor are suppressed, and these are only output again when a forward movement follows the backward movement, wherein the path of the forward movement corresponds to the path of the previously executed backward movement. Only after this are the signals of the incremental sensor output again as control signals.